The invention relates to coated spinel color pigments whose core is formed of a colored spinel and whose coating is a silicate material with a glassy structure. The invention also relates to the production of the coated spinel color pigments, as well as their use for the production of decorations with improved contour sharpness.
Color pigments for coloring and decorating porcelain, earthenware, and other ceramic products which are fired in the presence of glass frits above the softening point of the glass frit, must have high temperature stability, oxidation stability, and glaze stability. For the production of decorations with more or less fine patterns, decorative colors are generally used which essentially consist of a color pigment and a glass frit.
Because of their frequently brilliant colors, pigments of the spinel type have long been used to color glazes and as a component of decorative colors. Pigments which crystallize in a spinel type structure, e.g. Thenard blue (CoAl.sub.2 O.sub.4) and copper chromite (CuCr.sub.2 O.sub.4), are mostly very color-intensive pigments. Color pigments of the spinel type are usually essentially stable in regard to their color in the presence of an aggressive glass flux, but the so-called bleeding effect is disadvantageous. Because of the interaction of the pigment with the glass flux, the pigment is more or less dissolved, i.e. components of the pigment migrate into the surrounding glass flux and color it. This "bleeding" prevents the production of decorations with high contour sharpness. This effect is particularly noticeable for the Co-containing spinel CoAl.sub.2 O.sub.4.
In order to prevent the bleeding problem and/or other instabilities during firing-on, such pigments have already been encapsulated in clear crystals of glaze-stable substances as described in German AS 23 12 535. Therein is described a process of heating a mixture of zirconium oxide, silicon oxide, and finely divided Thenard blue, a light blue inclusion pigment. The inclusion of spinel pigments in glaze-stable, colorless zirconium silicate crystals is therefore possible, but only very weakly colored inclusion pigments were obtained in this way.
The object of the invention is therefore to create coated spinel pigments with a higher color intensity than the colored spinels encapsulated in zirconium silicate crystals. Compared to the uncoated spinel color pigments, the new pigments are resistant to bleeding or at least bleed less during firing-on, and thus allow the production of decorations with improved contour sharpness.